1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to an improved method and an implantable medical device for assessing a degree of pulmonary edema of a patient.
2. Description of the Prior Art
Today, in the modern society, heart diseases and/or conditions leading to an impaired heart function are a major problem entailing constantly increasing costs for medical services. For example, heart failure is a condition which affects thousands of people throughout the world. Congestive heart failure (CHF) is the inability of the heart to supply the body with an adequate amount of blood. Patients suffering from CHF are often afflicted by cardogenic pulmonary edema, which is caused by the accumulation of fluid in the lung interstitium and alveoli due to the fact the left ventricular venous return exceeds left ventricular cardiac output. That is, more fluids are transported to the lung region than from the lung region causing the accumulation of fluids in the lung region. Hence, pulmonary edema is a common effect of congestive heart failure (CHF) and CHF patients are often on diuretics to decrease the risk of this. However, diuretics can be difficult to titrate and the amount needed may differ from day to day. Accordingly, reliable and accurate information that can be used to assess a degree of pulmonary edema or amount of pulmonary fluid would be of great use. Such information should also be easy and convenient to obtain, should be sensitive to early changes of the pulmonary fluid, and should be obtained automatically. In this respect, the electrical bio-impedance and, in particular, the trans-thoracic impedance have been found to constitute an effective measure for identifying changes of different conditions of the body of a patient, such as incipient pulmonary edema and the progression of pulmonary edema due to CHF, i.e. the accumulation of fluids in the lung-region associated with pulmonary edema affects the thoracic impedance, or more specifically the DC impedance level, since the resistivity of the lung changes in accordance with a change of the ratio of fluid to air. The DC impedance level is negatively correlated with the amount of fluids in the lung. Studies have shown that hospitalization due to the development of acute CHF with the symptom pulmonary edema was preceded two or three weeks by a drop in the DC impedance by approximately 10-15%.
In light of this, many approaches have been suggested for providing information that can be used to assess a degree of pulmonary edema or amount of pulmonary fluid by utilizing the thoracic impedance. In U.S. Pat. No. 6,595,927 a method and system for diagnosing pulmonary congestion in a mammalian heart using trans-thoracic impedance is disclosed. The trans-thoracic impedance and a heart rate of a patient are measured and an activity sensor is used to sense a heart activity in order to determine an exercise level of the patient. The impedance change over time intervals between commencement and termination of exercise is used as a quantitative measure of the degree of pulmonary congestion. Another approach is shown in U.S. Pat. No. 6,104,949, where a method and system for diagnosing pulmonary congestion in a mammalian heart using trans-thoracic impedance is disclosed. The trans-thoracic impedance is measured and a body posture of the patient is sensed. Changes in posture is correlated with trans-thoracic impedance changes and the impedance change over a time interval after a posture change is used as a quantitative measure of the degree of congestive heart failure.
However, a problem associated with present methods for measuring the electrical bio-impedance and, in particular, the trans-thoracic impedance is the accurateness and reliability of the obtained signals since they are greatly affected by factors like the body position of the patient, patient activity levels, heart rate frequency, etc. For example, it has been found that the body position of the patient is of major importance with regard to the thoracic impedance. In addition, it has recently been found that the posture or position dependence also is of a significant magnitude regarding different positions even when the patient is lying down, for example, whether the patient is lying on a side or is lying on the back. A major reason is that an impedance measurement depends on the measurement vector, i.e. the vector between the nodes that the current is applied between and the vector the voltage is measured between. When the body shifts position, these vectors will change since the gravity will influence, for example, tissue between the nodes and how it moves. Tests performed on animals have shown that the trans-thoracic impedance may vary up to 20% depending on the position of the animal.
Accordingly, there is a need for an improved method and implantable medical device for assessing a degree of pulmonary edema of a patient or for obtaining reliable and accurate information for such an assessment in an automatic way.